This proposed research is designed to compare the retentive qualities of threaded pins, and undercut slots when used to make extensive restorations such as a full crown build-up. Dental amalgam and composite materials will be used for in vitro studies and amalgam for the in vivo phase. Extracted teeth and ivorine cylinders will be used to test the variables in slot design: depth, wall angle and distance from Dentino-enamel junction. These test data will be gathered under tension; compression and shear; and alternating stresses. From these data the best slot design will be determined. Using this slot design, the retention of slots and pins will be compared under tension; compression and shear; and alternating stresses. With the best slot design established, cusp replacement restorations will be made on two groups of teeth and the teeth rebuilt using this design and the threaded pins. All the teeth done in vitro will be tested for ultimate strength and from this data an evaluation of their respective merits made. The clinical restorations placed will be radiographed, photographed and evaluated for marginal seal on four separate occasions: baseline, and baseline plus 1, 2, and 3 years. A careful record of clinical failures will be kept and the type and reason for such failures recorded and categorized.